Impressive scientific and technological progress has recently been achieved in the area of organic light-emitting diodes (OLEDs), motivated by potential applications in a large variety of display technologies. OLEDs are “dual-injection” devices in which holes and electrons are injected from opposite electrodes into an active molecular/macromolecular medium to produce, via exciton decay, light emission. OLED responses are usually evaluated with respect to the following characteristics: luminance—light intensity per unit area, turn-on voltage—voltage required for a device to reach luminance of ˜1 cd/m2, and current efficiency—luminance per unit current density. To achieve optimum device performance, it has been thought desirable to have multilayer structures having discrete hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL) functions. The role of the HTL is not only to maximize hole injection from the anode (usually ITO—tin-doped indium oxide), but also to block efficiency-depleting electron overflow from, and to confine excitons within, the EML. With such multilayer structures, high-performance devices have been realized for small-molecule-based OLEDs fabricated via vacuum deposition. Typical small-molecule HTLs are triarylamine-based materials such as NPB or TPD (FIG. 1), which are known to have appreciable hole-transporting and electron-blocking/exciton-blocking capacity, because of their relatively high-lying LUMO levels and large HOMO-LUMO gaps.
For polymer-based LEDs (PLEDs), while single-layer polymer devices have obvious attractions such as ease of fabrication and large carrier mobilities, their performance is limited by inefficient hole injection from the ITO anode into the HOMO level of the EML polymer, as a result of, among other factors, the ITO work function (4.7 eV) and EML HOMO level (5.3-5.9 eV) mismatch. Compared to its small-molecule counterparts, a multilayer PLED device is far more challenging to fabricate due to the risk of partially dissolving a previous layer while depositing the next in solution casting processes. Examples of conventional PLED HTLs are p-doped conductive polymers such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS), polyaniline-camphorsulfonic acid (PANI-CSA), and polypyrrole-dodecylbenzene sulfonic acid (Ppy-BDSA). In most cases, these HTL films are cured at high temperatures (˜200° C.) after spin-coating, and thus rendered insoluble. These conventional HTLs have been shown to significantly enhance PLED anode hole injection (by increasing the anode work function and smoothing energetic dicontinuities) and device performance.
However, these HTLs also have serious drawbacks such as corrosion of the ITO anode, poor surface energy match with typical aromatic EMLs, and mediating the luminescence-degrading oxidative doping of polyfluorene EMLs. Furthermore, the question still remains as to whether PEDOT-PSS has the magnitude of electron-blocking capacity required for a truly high-performance PLED HTL. In addition to these more conventional conductive polymer HTLs, there has also been intense research activity focused on photo- or thermally crosslinkable and in situ polymerized PLED HTLs. Most of these crosslinkable HTLs require either high temperature baking (150-200° C.) or ultraviolet photochemical processing. Moreover, many of these crosslinked films suffer from microcracking due to volume shrinkage on crosslinking of the materials, which could lead to undesirable leakage currents in the PLED devices.